In this study, we discuss the consistence of measured and calculated TDR traces. The calculated traces are solutions of a time domain reflectometry (TDR) forward solver, an algorithm for computing the TDR trace for a given dielectric profile along a transmission line. An unambiguous and efficient forward solver is a prerequisite for a good solution of the inverse problem, i.e., to extract the spatial distribution of the dielectric properties along the transmission line from a TDR trace. To advance our understanding of TDR inversion, we proceeded in two steps: (1) design of a TDR head section with minimal disturbances on the signal and (2) searching for causes why measured and predicted TDR traces differ. Based on a first experiment with a three‐rod TDR probe of 100 cm length, we demonstrated that our TDR forward solver—like others presented in literature—approximate the measured TDR traces apparently well but not precisely enough for signal inversion. In a second experiment, using a two‐rod TDR probe of 70 cm length, we addressed the problem of non‐parallel transmission lines. We found that the influence of a non‐parallel installation is similar to an increase of the electrical conductivity in soil water but can be distinguished from this property. A third experiment reveals that lateral and longitudinal disturbances in the vicinity of a TDR probe are of minor importance. From the analysis of our experiments, we found that neither lateral disturbances nor non‐parallel rods are responsible for the deviations between calculated and measured traces. This analysis showed us that structure in the sampled medium affects the shape of the TDR traces. Since minor deviations are essential for TDR‐signal inversion, we need new concepts to handle the fuzziness between measurements and calculations.

Single Conductor Wave Guides Time domain reflectometry (TDR) probes consisting of one con-From the theory, we can derive some general properducting rod and a wave mode converter are an alternative configuraties of single conductor wave guides valid for coated and tion that overcomes some of the disadvantages of conventional probes. uncoated electrodes. At a given frequency f and in anWe examined four different single-rod probes (SRPs) and a two-rod environment with a homogeneous relative dielectric probe for sensitivity to a small and a large conductive scatterer in constant ε r , a single conductor wave guide is fully charactheir vicinity. The SRPs were assembled combining a small and large terized by k z , the number of full waves per meter along wave mode converter with an uncoated and coated rod. We found the rod. The longitudinal wave number k z is a result of that the volume sampled by SRPs is larger and more symmetric than in the case of a two-rod probe of equal size. A comparison of the a transcendental eigenvalue equation comprising f, ε r , mode converters showed a higher loss for the smaller converter but the electrode radius r e , the conductivity e of the eleconly a small difference concerning the spatial sensitivity. Coating the trode, the dielectric constant of the coating ε rd (if any), conducting rod with a high dielectric constant material reduces the and the coating thickness d (Eq. [A18] in the Appendix). spatial sensitivity. One of the SRPs and the two-rod probe wereFor a chosen frequency f and for a given setup, k z has calibrated in a sand tank (particle size 0.08-0.2 mm) with volumetric to be determined numerically.water content up to 0.35 m 3 m Ϫ3 . The calibration showed only small differences in the measured bulk dielectric constant between the sin-Wave Velocity and Attenuation gle-rod and the two-rod probe. Based on this study the SRP is a promising new tool for improved TDR measurement of soil moisture.

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